Organic materials have recently shown promise as the active layer in organic based thin film transistors and organic field effect transistors [see H. E. Katz, Z. Bao and S. L. Gilat, Acc. Chem. Res., 2001, 34, 5, 359]. Such devices have potential applications in smart cards, security tags and the switching element in flat panel displays. Organic materials are envisaged to have substantial cost advantages over their silicon analogues if they can be deposited from solution, as this enables a fast, large-area fabrication route.
The performance of the device is principally based upon the charge carrier mobility of the semi-conducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2V−1 s−1). In addition, it is important that the semi-conducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance.
A known compound which has been shown to be an effective p-type semiconductor for OFETs is pentacene [see S. F. Nelson, Y. Y. Lin, D. J. Gundlach and T. N. Jackson, Appl. Phys. Lett., 1998, 72, 1854]. When deposited as a thin film by vacuum deposition, it is shown to have carrier mobilities in excess of 1 cm2 V−1 s−1 with very high current on/off ratios greater than 106. However, vacuum deposition is an expensive processing technique that is unsuitable for the fabrication of large-area films.
Regular poly(3-hexylthiophene) has been reported with charge carrier mobility between 1×10−5 and 4.5×10−2 cm2 V−1 s−1, but with a rather low current on/off ratio between 10 and 103 [see Z. Bao et al., Appl. Phys. Lett. 1997, 78, 2184]. In general, poly(3-alkylthiophenes) show improved solubility and are able to be solution processed to fabricate large area films. However, poly(3-alkylthiophenes) have relatively low ionisation potentials and are susceptible to doping in air [see H. Sirringhaus et al., Adv. Solid State Phys. 1999, 39, 101].
It was an aim of the present invention to provide new organic materials for use as semiconductors or charge transport materials, which are easy to synthesise, have high charge mobility and good processability. The materials should be easily processable to form thin and large-area films for use in semiconductor devices. Another aim as to extend the pool of semiconducting materials available to the expert. Other aims of the invention are immediately evident to those skilled in the art from the following description.
It was found that the above aims can be achieved by providing reactive mesogenic compounds according to the present invention. They consist of a central mesogenic core comprising two or more thiophene rings, and optionally one or more phenylene rings that form a conjugated system together with the thiophene rings, said mesogenic core being linked, optionally via a spacer group, to one or more reactive groups. The compounds can induce or enhance liquid crystal phases or are liquid crystalline themselves. They can be oriented in their mesophase and the polymerisable group can be polymerised or crosslinked in situ to form polymer films with a high degree of order, thus yielding improved semiconductor materials with high stability and high charge carrier mobility.
A further aspect of the invention relates to liquid crystal polymers, in particular liquid crystal side chain polymers obtained from the reactive mesogenic compounds according to the present invention, which are then further processed e.g. from solution as thin layers for use in semiconductor devices.
Reactive mesogenic compounds for semiconducting applications have been described in WO 03/006468 A2, EP 1 275 650 A2, EP 1 275 652 A2, EP 1 279 690 A1, EP 1 279 691 A1, EP 1 279 689 A2, EP 1 284 258 A2, EP 1 318 185 A1, EP 1 300 430 A1, EP 1 357 163 A1, EP 1 398 336 A1 and 1. McCulloch et al., J. Mater. Chem. 2003, Vol. 13, p. 2436-2444.